I have never used a TTL converter before and have only read Scotts article on it http://www.uwphotographyguide.com/ttl-converters so I'm not sure I fully understand why I need it. If I understand it correctly DTTL will only work with Nikon strobes and it won't work with more then one, right? In order to be able to use TTL with other strobe brands I will need a TTL converter so that the strobe and camera can communicate. The D100 is rather old, will it be able to understand the output from Sea&Sea TTL converter (#50117)?[/quote]

In your old system you had a Nikon strobe in a housing that talked directly to the Nikon camera. That's why TTL worked. Unfortunately, Nikon (and Canon) will not tell other strobe companies exactly how DTTL works so guys like Matthias at Heinrichs-Weikamp essentially reverse engineered the protocol and built a small circuit board that translates the Nikon signals into signals that the strobes can understand. I believe that the Sea and Sea converters essentially use H-W converter circuits but packaged in an external device. H-W converters are labeled as compatible. I use an H-W converter to trigger Z240 strobes so all should be good if you go that route. It is not clear from Sea and Sea if their TTL converter N will work with the D100. They suggest that it will for the D200 and later but I can't find anyone who knows, so if you buy the TTL converter from S&S make sure you can send it back if it doesn't work.